It is known that diverse methods are employed for controlling the microstructure and service properties of materials, particularly of metals and metal alloys. Some of the methods include heat treatments, such as annealing and quenching, various mechanical operations involving plastic deformation, such as cold working and shot peening, and combination methods such as hot working.
The use of an electric current flow in conjunction with some of the above methods is known, primarily as a means of effecting workpiece heating. For example, Ruben (U.S. Pat. No. 1,683,209) uses a current flow along a wire to heat it in the presence of a magnetic field to increase its conductivity. Oshida (U.S. Pat. No. 3,986,898) use an electric current to heat a workpiece in order to induce controlled internal oxidation and thus improve the high temperature performance of a material. The electric current can be applied directly to the workpiece as in the above examples, or to a working fluid surrounding the workpiece. Solokian (U.S. Pat. No. 2,349,767) uses a three-phase AC current to heat a salt bath used to harden high speed steel. In addition, Solokian uses a d.c. current to scavenge metal impurities from the salt bath when workpieces are not present. In some instances, it has been found advantageous to use high frequency alternating current flows, in contrast to the d.c. or low frequency a.c. described previously. By using high frequencies, especially in conjunction with short processing times, the current flow can be confined to the surface of the workpiece. Roberds (U.S. Pat. No. 2,395,195) uses high frequency current flow to harden either the outer or inner surface of hollow cylindrical objects. Rudd (U.S. Pat. No. 4,215,259) combines rapid high frequency current heating with natural conduction cooling to harden materials such as steel by quenching. Froehlich (U.S. Pat. No. 5,073,212) uses a pulsed heat source, which may either be a laser or a high frequency induced current, to harden the root of turbine blades through a martensitic transformation. In all of the examples cited above, the disclosures teach the use of an electrical current as a means of heating.
In other examples, electrical current in the form of an electrical discharge is used to modify the surface properties of materials and components. Fruth (U.S. Pat. No. 1,966,496) uses an electrical spark, arc, and corona discharges to harden the surface of non-ferrous metals. Blaskowski (U.S. Pat. No. 3,360,630) uses a spark discharge followed by mechanical burnishing to harden the surface of metals. These disclosures illustrate the electrical current is flowing essentially normally to the surface of the conductor, and furthermore the current is flowing from the conductor into a normally insulating medium at the point where modification of the material is taking place.
An early use of an electrical current in conjunction with quenching is described by Sedgwick (U.S. Pat. No. 400,366). An electric current is applied to the workpiece as it is plunged into the coolant bath. The formed gas bubble formation breaks up the clinging steam bubbles, and hence the heat transfer to the coolant is improved. In this case, the electrical current is flowing essentially normally to the surface of the conductor; and, furthermore, the current is flowing from the conductor into the surrounding liquid medium.
A very different use of electrical current for improving the properties of conducting materials has been described recently, in which the electric current itself, or its concurrent electric field, causes an advantageous alignment of the crystal structure of the material at high temperatures. Giancoloa (U.S. Pat. No. 5,073,209) heats a conducting material to a temperature such that physical rearrangement of the atoms is possible by applying an electrical current flow. The sample is then cooled while the electrical current is continued in order to obtain conducting materials with an improved conductivity and superconducting materials with a high critical temperature. McKannan et al. (U.S. Pat. No. 5,080,726), heats the central portion of superalloy while passing an electrical current along its length and then applies directional cooling to obtain materials with an ordered microstructure.
Furthermore, transparent electrically conductive coatings have been known for a variety of applications, including window heaters, electrodes for displays, light emitting devices, light detectors, photovoltaic solar cells, light triggered semiconducting devices, and invisible burglar alarms for display windows. The most popular transparent coatings are fabricated from certain metal oxides, in particular, indium oxide, tin oxide, and zinc oxide. Enhanced conductivity is often obtained with the addition of a doping material, i.e., tin, in the case of indium oxide; antimony, in the case of tin oxide; and aluminum, in the case of zinc oxide. While a variety of techniques exist to deposit these coatings, they can generally be divided into two groups. The first group contains high deposition rate techniques such as spray pyrolysis and chemical vapor deposition. The conductivity of the coatings produced by these techniques is generally slower than that obtainable by the second group. The second group includes sputtering, evaporation, and activated reactive evaporation. These techniques can produce high conductivity coatings, but the deposition rate is very low and the cost very high.
The vacuum arc technique to produce coatings has been known for some time. The technique is widely used for metallurgical coatings, especially for TiN coatings on cutting tools, which extend their service lifetime considerably. Optical coatings have been recently developed by using this technique. One difficulty with the vacuum arc is that the cathode spots produce a spray of liquid droplets, or macroparticles, of the cathode material. While some macroparticle inclusion can be tolerated in metallurgical applications, they can be extremely deleterious in optical and electronic applications.
In vacuum arcing, an electrical arc is sustained between two conductive electrodes in a chamber which is either evacuated or has a low pressure process gas. The arc tends to be constricted at minute areas of the cathode surface, known as cathode spots, which reach very high temperatures and produce copious quantities of vaporized cathode material, which becomes highly ionized by the action of the arc current passing through it. The resulting metal vapor plasma jet will produce a metal coating on surfaces on which it impinges in an evacuated chamber. If a reactive process gas is present, a compound of the cathode material and the process gas may be deposited.
It is known that macroparticle contamination can be virtually eliminated by passing the plasma beam through a curved duct with a magnetic field parallel to duct. The magnetic field bends the plasma beam so that it follows the curvature of the duct, while macroparticles are not affected by the magnetic field and collide with the walls of the duct, and, for the most part, adhere thereon. Vacuum arcing in conjunction with macroparticle filters have produced optical quality Al2O3 and TiO2 coatings.
The most popular transparent conductors, Indium (Tin) Oxide, and Tin (Antimony) Oxide, are based on low melting-point metals. Low melting-point metals are particularly prone to producing a prodigious amount of macroparticles. Furthermore, the materials are often difficult to obtain in forms suitable for fabricating a cathode. Also, some of the materials, such as Indium, are sufficiently expensive to justify efforts to fully utilize all the material. In addition to problems of cathode fabrication, process parameters must be optimized for good coating conductivity.
What is needed is a transparent conductive coating with reduced macroparticles and improved microstructure. What is also needed is a method to influence the microstructure that does not require high temperatures to process solid materials nor require the use of directional cooling when processing liquid materials. What is also needed is an economical way of producing transparent conductive coatings having a high conductivity at a fast deposition rate.